This invention relates to compositions for curing epoxide resins, to curable mixtures of these compositions and epoxide resins, and to cured products obtained by curing the aforesaid mixture.
It is known that epoxide resins, i.e., substances containing on average more than one 1,2-epoxide group per molecule, may be cured by reaction with various classes of substances to form cross-linked, infusible, insoluble products having valuable technical properties. An important class of curing agents comprises aromatic, aliphatic, heterocyclic, and cycloaliphatic polyamines, including aminoamides; by "polyamine" is meant an amine having at least three hydrogen atoms directly attached to amino nitrogen atoms.
The chief drawback with the polyamines at present in use is that they often cure the resin only slowly, particularly when aromatic amines are employed. The use of accelerators alleviates this drawback to some extent but the accelerating effect imparted by those hitherto available is relatively modest.
It is known from British patent specification No. 1,314,561 that salts of trifluoromethanesulfonic acid induce cationic polymerization of epoxide resins on heating the salt and resin together. In general, the reactants were heated at a temperature of from about 50.degree. to 218.degree. C. Gelation at 50.degree. took, according to the Examples, 4 days in one case but in the remaining cases 14, 14, 33, 52, 35, 58, 70, and 49 days. In the experiment carried out at room temperature, the resin-catalyst mixture was still liquid after 1 year. In other experiments, temperatures of 132.degree., 149.degree., and 204.degree. C are employed. It can be seen that, at room temperature or a little above, the trifluoromethanesulfonate had a negligible effect on the epoxide resin. Although the use at 132.degree. C of a polycarboxylic acid anhydride as a co-curative is illustrated, it was not to be expected from the disclosures of the aforesaid British patent that a salt of a trifluoromethanesulfonic acid would have the valuable property of strongly accelerating the curing of epoxide resins by certain polyamines, even without the application of heat.
U.S. Pat. No. 3,907,706, a subsequent patent to the same assignee, disclosed latent epoxide resin compositions containing both a metal salt of trifluoromethanesulfonic acid and a thermally decomposable ester reaction product of a tertiary alkyl alcohol and an acid that forms a chelation complex with the metal ion of the metal salt. To retard, if desired, the activity of the catalyst system, there may be included a buffering compound. In the list of suitable buffering agents, an aliphatic polyamine is named. Again, it would not have been expected that a salt of trifluoromethanesulfonic acid would accelerate the curing of epoxide resins by certain polyamines.
We have now found that salts of trifluoromethanesulfonic acid very markedly accelerate the cure of epoxide resins with amines.